a) Field of the invention
The present invention relates to an optical microscope, and more specifically to a highly systemized stereomicroscope.
b) Description of the prior art
There are available conventional most systemized high performance stereomicroscopes which are exemplified by the microscope disclosed by Japanese Patent Preliminary Publication No. Sho 55-113014 (hereinafter referred to as prior art No. 1) and that disclosed by Japanese Patent Preliminary Publication No. Sho 60-227214 (hereinafter referred to as prior art No. 2).
FIG. 1 illustrates an optical system of the microscope according to prior art No. 1, wherein a light beam emitted from a light source 1 of a first optical system I is linearly polarized by a polarizer 2, reflected by a polarizing beam splitter 3 which functions to reflect light in a polarizing direction thereof and allow light in a direction perpendicular to the polarizing direction to pass therethrough, and allowed to transmit through a variable magnification system 4 and an objective lens 5, whereafter the light beam is circularly polarized by a quarter wavelength plate 6 and illuminates a sample S. The circularly polarized light beam is reflected by the sample S, linearly polarized by the quarter wavelength plate 6 in the direction perpendicular to the polarized direction at the stage of the illumination, and allowed to pass through the objective lens 5 and another variable magnification system 4' of a second optical system II, whereafter the light beam attains to another beam splitter 3'. This polarizing beam splitter 3' is oriented in the same direction as that of the polarizing beam splitter 3 of the first optical system I so that the reflected light beam is allowed to transmit through the polarizing beam splitter 3' of the second optical system II and attains to an analyzer 7'. Since this analyzer 7' is oriented so that it allows to pass the linearly polarized light beam incident thereon, the reflected light beam which has passed through the analyzer 7' of the second optical system II is imaged by an eyepiece lens system 8' and can be observed therethrough.
A light beam emitted from a light source 1' of the second optical system II is, like the light beam emitted from the light source 1 of the first optical system I, imaged by an eyepiece lens 8 disposed in the first optical system I.
On the other hand, a polarizer 2' is oriented in the second optical system II in the same direction as that of the polarizer 2 of the first optical system so that rays which are reflected by the surfaces of the variable magnification system 4' of the second optical system II, the objective lens and so on, out of linearly polarized rays emitted from a light source 1' of the second optical system II and allowed to transmit through the polarizer 2' of the second optical system II, are perpendicular to the oscillating direction of the analyzer 7' of the second optical system II, whereby a light beam emitted from the light source 1' of the second optical system II is cut off by the polarizing beam splitter 3' and the analyzer 7' of the second optical system, and cannot attain to an image surface of the second optical system II. Since the analyzer 7 of the first optical system I functions to cut off rays reflected by lens surfaces, the microscope according to the prior art No. 1 provides an image which is to be observed with high contrast.
Further, in a stereomicroscope according to the prior art No. 2 which comprises the Gallelian optical system described above and a coaxial vertical illumination system, the quarter wavelength plate 6, the objective lens system 5, the variable magnification systems 4 and 4', a coaxial vertical illumination system EI (1, 2, 3, 7; 1', 2', 3', 7'), a photographing system PH, the eyepiece lens systems 8 and 8', and an intermediate tube allowing simultaneous observation by two microscopists are designed as independent units respectively as illustrated in FIG. 2 for permitting optional combinations of these units in conjunction with microscopy modes desired by users and enhancement in operability as well as convenience of the combinations.
In the conventional stereomicroscope described above, the coaxial vertical illumination unit is placed right over the objective lens system including the variable magnification systems (on the side of the eyepiece lens systems), the photographing unit and the simultaneous microscopy unit are set on the coaxial vertical illumination unit, and the eyepiece lens units including imaging systems are arranged on the simultaneous microscopy unit and the photographing unit. This configuration provides, first of all, a merit to assure a smaller loss of quantity of axial vertical illumination light. In other words, this configuration makes it possible to allow almost all of the rays which are reflected by the beam splitters 3 and 3' to transmit through the variable magnification systems 4, 4' and the objective lens unit 5 for illuminating a sample S. Further, since the objective lens unit 5 (including the variable magnification systems 4, 4') is located right under the coaxial vertical illumination unit, rays reflected by the lens surfaces are cut off effectively by the polarizers built in the beam splitters. Accordingly, it is unnecessary to arrange a reflected light cut-off filter in any one of the units located over the polarizers. Consequently, it is possible to use an observation unit for a transmission illumination mode with no additional device or obtain a highly systemized stereomicroscope.
When the intermediate tube, the photographing unit, etc. are piled over the coaxial vertical illumination unit, however, the conventional stereomicroscope poses a problem that an eye point EP is made higher than required as illustrated in FIG. 3A. In order to solve this problem, it was obliged to prepare a special tube which has an optical path modified so as to direct an afocal light beam coming from the objective lens system 5 downward as illustrated in FIG. 3B for obtaining an eye point located at a predetermined height. In FIG. 3A illustrating an example of a configuration of the ordinary type eyepiece tube, the reference symbol a represents a distance in the horizontal direction as measured from an optical axis to the eye point EP and the reference symbol b designates a distance in the vertical direction as measured from a base plane BP of the eyepiece tube unit to the eye point EP. Further, since the eyepiece tube unit uses an imaging lens IL which has a focal length shorter than the infinite focal length of the ordinary type objective lens system for microscopes, the eye point EP is inevitably located close to the optical axis of the objective lens system 5, thereby posing a problem that a microscopist must assume an unnatural posture (incline an upper half of his body forward) when the stereomicroscope has a large sample stage. In industries of LC's and LCD's wherein large wafers, liquid crystal panels and so on are manufactured in these days, for example, a stereomicroscope incorporating a large sample stage prolongs the distance a as indicated by a reference symbol a' in FIG. 4B and produces great inconvenience for inspectors of the IC's and LSD's. Since the coaxial vertical illumination unit is disposed right over the variable magnification units 4, 4' and the objective lens unit 5 in the configuration of the stereomicroscope described above, location of the eye point EP is enhanced and operability of the stereomicroscope is remarkably degraded when the microscope system is complicated by piling the photographing unit and the eyepiece tube unit over the coaxial vertical illumination unit.
Furthermore, since the coaxial vertical illumination unit prolongs the optical path length in the stereomicroscope, the afocal light beam coming from the objective lens system is expanded especially when the intermediate tube is interposed for photographing. Accordingly, it was conventionally obliged to prepare imaging lenses IL having a large diameter and use a large prism in the eyepiece tube unit or reduce a real field number on an intermediate image surface for observation. In this case, it is general to cut off rays to attain to marginal portions of a visual field and expand light beams to attain to central portions of the visual fields, or to perform contraction and relaying of a light beam by disposing relay lenses RL before and after the beam splitters 3, 3' arranged in the coaxial vertical illumination unit (see FIG. 5). However, this contraction-relaying method magnifies causes magnification of intermediate image, thereby producing inconvenience that an image is observed at different magnifications even with the same eyepiece tube dependently on whether or not the coaxial vertical illumination is used.
Moreover, when the coaxial vertical illumination mode is switched to the transmission illumination mode while the stereomicroscope is equipped with coaxial vertical illumination unit, there are posed problems that a quantity of the transmitted illumination light for forming an image is reduced to approximately 30% by the beam splitters 3, 3' and analyzers 7, 7' arranged in the first optical system I and the second optical system II respectively, and that flare unwanted for observation is produced by the beam splitters and the analyzers. When the illumination modes are switched as described above, the real field number in the transmission illumination mode restricts that on the side of the coaxial vertical illumination unit, thereby making it impossible to fully utilize the visual field of the stereomicroscope which should originally be large.
Moreover, it is theoretically possible with ease to observe an image formed with a transmitted polarized light beam through the stereomicroscope which is equipped with the coaxial vertical illumination unit by removing the quarter wavelength plate and setting a polarizer instead on the light source side of the sample S, but it is practically necessary for such observation to set the polarizing planes of the analyzers 7, 7' arranged in the coaxial vertical illumination unit accurately perpendicularly to the oscillating direction of the incident light beam. In addition, it is difficult to observe a correct image since polarized light produced by the sample S is further polarized due to shaping and assembling errors as well as internal deformation, radii of curvature, etc. of the objective lens 5 and the variable magnification lens 4. In such a case, it is obliged to arrange an analyzer in place of the quarter wavelength plate in the objective lens system 5 after dismounting the coaxial vertical illumination unit, thereby making it practically impossible to perform the so-called quick switching between the transmission-polarization illumination mode and the coaxial vertical illumination mode.